Water-absorbent resins are widely used in hygienic materials such as disposable diapers and sanitary napkins; daily commodities such as pet sheets; water absorbing sheets for food products; industrial materials such as water blocking materials for cables; water retention agents for greening/agriculture/horticulture; and the like.
Hygienic materials such as disposable diapers and sanitary napkins are generally constituted with a top sheet, a back sheet, a hot melt adhesive, an elastic material, a water-absorbent resin and a pulp fiber, various synthetic resins and modifiers are used. Therefore, an odor originating from raw material components is perceived from the hygienic materials, in some cases. Since these hygienic materials are put on the human body, the odor makes users uncomfortable even if it is subtle and, therefore, it is desired to develop an odor-free material.
Among constituent materials of these hygienic materials, the water-absorbent resin has a subtle odor originating from the substances used in the production process, and since the odor tends to emit upon water absorption, it is considered to be desirable to reduce the odor.
As water-absorbent resins used for hygienic materials, for example, a partially-neutralized product of polyacrylic acid, a neutralized product of a starch-acrylic acid graft polymer, a hydrolysate of a starch-acrylonitrile graft copolymer, a saponified product of a vinyl acetate-acrylic acid ester copolymer are known.
As methods for producing such water-absorbent resins, an aqueous polymerization method and a reversed-phase suspension polymerization method are known. In a case where a water-absorbent resin is produced by a reversed-phase suspension polymerization method in which polymerization is performed by suspending a water-soluble monomer in a dispersion medium, a major cause of the odor is considered to originate from the dispersion medium.
As conventional methods for producing the water-absorbent resin by a reversed-phase suspension polymerization method, known are a method of polymerizing an aqueous solution of α,β-unsaturated carboxylic acid and alkali metal salt thereof in a petroleum hydrocarbon solvent using a radical polymerization initiator in the presence or absence of a internal-crosslinking agent in which a sucrose fatty acid ester is used as a protective colloid agent (see Patent Document 1), and a method of polymerizing a 25% by mass or more of aqueous solution of an α,β-unsaturated carboxylic acid and alkali metal salt thereof in a petroleum hydrocarbon solvent using a radical polymerization initiator in the presence or absence of a internal-crosslinking agent in which a polyglycerol fatty acid ester with an HLB of 2 to 16 is used as a surfactant (see Patent Document 2). However, these production methods do not focus on reduction of an odor, and thus odors of the resultant water-absorbent resins are not sufficiently low.